1. Field of the Invention
The present invention relates to an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer and, more particularly, to an oa-TOF mass spectrometer which, if the ion source is contaminated with a sample, can remove the contaminant in a short time and resume measurements.
2. Description of Related Art
A mass spectrometer is an instrument in which ions created from a sample are made to travel through a vacuum. During the process of the flight, ions having different masses are separated and recorded as a spectrum. Known types of mass spectrometers include: magnetic mass spectrometer in which ions are dispersed according to mass using a sector magnetic field; quadrupole mass spectrometer (QMS) for sorting ions (filtering) according to mass using quadrupole electrodes; and time-of-fight mass spectrometer (TOFMS) for separating ions by making use of variations in time of flight due to different masses.
Of these mass spectrometers, magnetic mass spectrometer and QMS are adapted for ion sources that create ions continuously. On the other hand, TOFMS is suitable for ion sources that create pulsed ions. Accordingly, if one attempts to use a continuous ion source for TOFMS, some contrivance is necessary for utilization of the ion source. The orthogonal acceleration time-of-flight mass spectrometer (oa-TOFMS) is one example of TOFMS designed to emit pulsed ions from a continuous ion source.
A typical configuration of oa-TOFMS is shown in FIG. 1A. This instrument has a continuous atmospheric-pressure ion source 1 (such as electrospray ionization (ESI) ion source or inductively coupled plasma (ICP) ion source), differentially pumped walls 10a, 10b consisting of first and second partition walls and a vacuum pump (not shown), a first orifice 2 formed in the first partition wall of the differentially pumped walls 10a, 10b, a ring lens 3 placed within the differentially pumped walls 10a, 10b, a second orifice 4 formed in the second partition wall forming the differentially pumped walls 10a, 10b, an intermediate chamber 11 of a somewhat low degree of vacuum where an ion guide 5 is placed, lenses 6 consisting of focusing lenses and deflectors, a launcher 7 consisting of an ion repeller plate 7a and accelerating lenses (grids) 7b, a reflector 8 for reflecting ions, and a measuring chamber 13 of a high degree of vacuum where components forming the ion optics, such as an ion detector 9, are placed.
The various portions have the following degrees of vacuum. The degree of vacuum of the atmospheric-pressure ion source 1 is 0.1 MPa (atmospheric pressure). The degree of vacuum of the intermediate chamber 11 is in a range from 10−1 to 10−4 Pa. The degree of vacuum of the measuring chamber 13 is of the order of 10−5 Pa or better.
In this configuration, ions generated from the sample in the atmospheric-pressure ion source 1 are first introduced into the differentially pumped walls 10a, 10b through the first orifice 2. The ions tending to diffuse within the differentially pumped walls 10a, 10b are focused by the ring lens 3 in the walls 10. Then, the ions are admitted through the second orifice 4 into the intermediate chamber 11, where the ions are made uniform in kinetic energy. The ion beam diameter is reduced by an RF electric field produced by the ion guide 5. The ions are then guided into the high-vacuum measuring chamber 13. The partition wall that partitions the intermediate chamber 11 and the measuring chamber 13 from each other is provided with a third orifice 12 that places both chambers in communication with each other. This third orifice 12 shapes the ions that are guided in by the ion guide 5 into an ion beam of a given diameter (e.g., about 0.3 mm). The ion beam is then passed into the measuring chamber 13.
On the other hand, as shown in FIG. 1B, in an oa-TOFMS instrument having the continuous ion source 1 (such as electron impact (EI) ion source, chemical ionization (CI) ion source, field desorption (FD) ion source, or fast atom bombardment (FAB) ion source) in its intermediate chamber 11, ions produced in the ion source 1 pass through a focus lens 1′ and an orifice 12 and are introduced into the measurement chamber 13.
These various ion sources roughly have the following degrees of vacuum. The degrees of vacuum of EI ion sources are 10−2 to 10−3 Pa. The degrees of vacuum of CI ion sources are 5×10−2 to 5×10−3 Pa. The degrees of vacuum of FD ion sources are of the order of 10−4 Pa. The degrees of vacuum of FAB ion sources are of the order of 10−3 Pa.
The lenses 6 consisting of the focusing lenses and deflectors are installed at the entrance of the measuring chamber 13. The ion beam entering the measuring chamber 13 is corrected for diffusion and deflection by the lenses 6 and introduced into the launcher 7. Installed inside the launcher 7 are the ion reservoir and accelerating lenses arrayed in a direction orthogonal to the axis of the ion reservoir. In this ion reservoir, an ion repeller plate is disposed opposite to the grids.
The ion beam first travels straight toward the ion reservoir 17 that is located among the repeller plate 14, grids 15, and accelerating lenses 16 as shown in FIG. 2. The ion beam 18 moving straight through the ion reservoir 17 and having a given length is accelerated in a pulsed manner in a direction (X-axis direction) vertical to the direction (Y-axis direction) along which the ion beam 18 enters, by applying a pulsed accelerating voltage to the repeller plate 14. This forms pulsed ions 19 which begin to travel toward a reflector (not shown) mounted opposite to the ion reservoir 17.
The ions accelerated in the vertical direction travel in a slightly oblique direction slightly deviating from the X-axis direction because the velocity in the Y-axis direction assumed on entering the measuring chamber 13 and the velocity in the X-axis direction orthogonal to the Y-axis direction are combined. The latter velocity is given by the repeller plate, grids, and accelerating lenses. The ions are reflected by the reflector 8 and arrive at the ion detector 9.
When the ions are being accelerated, the same potential difference acts on every ion regardless of the masses of the individual ions. Therefore, lighter ions have greater velocities and vice versa. As a consequence, variations in ion mass appear as variations in arrival time taken to reach the ion detector 9. Variations in ion mass can be separated as variations in ion flight time.
In this way, the continuous ion source can be applied to TOFMS adapted for a pulsed ion source by accelerating the ion beam created from the continuous ion source 1 in a pulsed manner by the launcher 7 consisting of the repeller plate, grids, and accelerating lenses.
In oa-TOFMS, the kinetic energy of ions made to enter the ion reservoir is normally set to a very small value of less than 50 eV. Therefore, oa-TOFMS is affected much more by charging of the electrodes than the magnetic mass spectrometer. As a result, if an area ranging from the external ion source to the ion reservoir is charged at all, the ion beam introduced into the ion reservoir is deflected and tilted as shown in FIG. 3. This deteriorates the resolution and sensitivity of oa-TOFMS.
Such charging can occur quite easily by adhesion of organics to the surfaces of the electrodes, the organics being residues of the sample ions. Especially, this phenomenon occurs quite easily in measurements using cold-spray ionization mass spectrometry that is one method of ESI (see Japanese Patent No. 3137953) or inductively coupled plasma-mass spectrometry (ICP-MS) because the concentration of the sample is very high and the components around the ion trajectory within the vacuum region on the side of the ion source are often contaminated in a short time.
A conventional measure for eliminating this problem consists of halting the operation of the oa-TOFMS, breaking the vacuum, taking components located around the ion trajectory within the vacuum region on the side of the ion source into the atmosphere, and cleaning the components.
If the cleaning is done by this method and then the components are mounted again, a waiting time of from about half to full day is necessary until the degree of vacuum of the oa-TOFMS is recovered. During this time interval, the instrument cannot be used.
On the other hand, in a normal gas chromatograph-mass spectrometer (GC-MS), a system having an isolation valve and a preliminary evacuation chamber has been already put into practical use such that contamination of the ion source can be removed while the vacuum in the mass spectrometer is maintained (see Japanese Patent Laid-Open No. 2004-134321 and Japanese Patent Laid-Open No. 2004-139911).